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Measuring the oxygen positions in an ultra thin film by XRD

By Jennifer Fowlie

Based on article published in Nano Letters

X-ray diffraction is a ubiquitous tool used in many areas of physical sciences for determining the structure of matter. There are two particular experimental challenges in measuring an x-ray diffraction signal; samples that are of too small a volume and atoms that are of too small a radius. These two challenges coincide in the field of oxide thin films where the oxygen positions are of great importance in determining the physical properties and the sample film thickness can be as little as one atomic layer thick.

In our recent thickness-dependent study we were able to obtain a diffraction signal from the sublattice of oxygen octahedra in perovskite films as thin as 2 nm. Following an established method, we could reconstruct the octahedral tilt system as well as the tilt and rotation magnitudes. These can be critical parameters in, for example, perovskite nickelates where the Ni-O-Ni bond angle characterises the stability of the low temperature insulating state of the material.

We uncovered strikingly different behaviour between the film/substrate combinations studied. Notably, it appears that at interfaces with a high level of chemical and structural discontinuity there is a large variation of the oxygen network as the film thickness is increased from 2 nm to 6 nm. This demonstrated ability of using layer thickness as a tool to control the atomic positions on such a fine scale is certainly appealing from a materials engineering standpoint and, with the added applicability of x-ray diffraction as a probe towards the atomic limit, a future of functional tailor-made materials is within reach.

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